Wet oxidation is a well known technology for the destruction of pollutants in wastewater. The process involves treatment of the wastewater with an oxidant, generally molecular oxygen from an oxygen-containing gas, at elevated temperatures and pressures.
Wet oxidation at temperatures below the critical temperature of water, 374.degree. C., is termed subcritical wet oxidation. The subcritical system operates at sufficient pressure to maintain a liquid water phase. This is a commercially used treatment process employed for conditioning sewage sludge, the oxidation of caustic sulfide wastes, regeneration of powdered activated carbon, and the oxidation of chemical production wastewaters, to name only a few applications. Complete removal of all pollutants from wastewater by wet oxidation at subcritical conditions generally cannot be achieved. Thus, subcritical wet oxidation may not provide the high degree of treatment required by governmental regulations for direct discharge of the oxidized effluent to the environment. A number of U.S. Patents disclose processes designed to achieve complete removal of pollutants from wastewaters by wet oxidation.
Morgan in U.S. Pat. Nos. 3,849,536 and 3,917,460 discloses a process and apparatus for wet combustion of pulping liquors. The wet oxidation occurs at subcritical conditions in two stages within a single vessel with air from the first stage oxidation separated prior to introduction of air for oxidation in the second stage.
In Thiel et al. U.S. Pat. No. 4,141,829 describe a subcritical wet oxidation with a catalytic vapor phase oxidation of volatile organics in the offgases.
Bodenbenner et al. in U.S. Pat. No. 4,347,144 disclose a two-stage subcritical wet oxidation process where a majority of the wastewater COD is removed in the first stage and an oxidizing agent stronger than oxygen is added to the liquid phase in the second stage for complete COD removal.
Horak et al. in U.S. Pat. No. 4,525,283 describe a catalytic two-stage subcritical wet oxidation system where both metal ions and carbonaceous materials are employed as catalysts for wet oxidation of wastewater.
In Cederquist U.S. Pat. No. 4,604,957 discloses a subcritical two-stage wet oxidation process where a large excess of oxygen is used in the second stage oxidation and then the oxygen is recycled to the first stage where oxidation of the majority (70-90%) of the organic content of the wastewater occurs.
Wet oxidation at temperatures above the critical temperature of water, 374.degree. C., is termed supercritical wet oxidation. Above this critical temperature, no liquid phase is present, regardless of how much pressure is applied to the system. The properties of water in the supercritical phase are quite different from those in the liquid phase. Oxygen is miscible with supercritical water and thus oxidation reactions occur quite rapidly at these high temperatures, with an extremely high degree of destruction obtained for pollutants.
Supercritical wet oxidation is, however, a new technology with a number of operational problems involved. The high operating temperatures and pressures result in extreme corrosion problems as well as requiring expensive thick-walled piping and reaction vessels. Inorganic salts are not nearly as soluble in supercritical water as in liquid water, and these salts can precipitate under supercritical wet oxidation conditions, causing plugging of the equipment.
Wet oxidation of relatively high strength wastewaters, with chemical oxygen demand (COD) of about 20 grams per liter or greater, presents a most difficult problem, particularly for supercritical wet oxidation treatment. In the subcritical wet oxidation process, the exothermic oxidation reactions occur in an aqueous matrix. The heat released raises the temperature of the aqueous medium to some selected operating temperature. However, to initiate the oxidation reactions, the influent stream has to be preheated to a minimum temperature where the rate of oxidation and extent of oxidation in the wet oxidation reactor are sufficiently high that the selected operating temperature can be achieved. By balancing the heat exchange between oxidized effluent and influent feed, and the heat input, the selected operating temperature is reached. If excess heat is produced in the reactor vessel, this can be removed from the system by evaporation of liquid water within the reactor and removal of the steam produced.
In a supercritical wet oxidation system, the exothermic heat of oxidation released is utilized in raising the temperature of the supercritical water phase. If a high strength wastewater, having a COD of about 20 g/l or more, is introduced into the reactor with excess oxidizer, the oxidation reaction produces so much heat that the temperature of the contents rises, uncontrollably, to very high levels, e.g. 500.degree.-600.degree. C. A phenomenon referred to as a supercritical or diffusion flame has been observed in supercritical wet oxidation. The flame can ignite at temperatures near 500.degree. C. in the presence of oxidizable materials, e.g. a six mole percent concentration of fuel such as methane or methanol. Ignition of a supercritical flame is accompanied by extremely high temperatures. At supercritical conditions no liquid water phase is present to be vaporized to steam to remove heat from the reactor vessel.
A supercritical wet oxidation unit is constucted with design parameters for operation at a maximum temperature (e.g. 550.degree. C.) and a maximum pressure (e.g. 350 atm). If a supercritical flame ignites in the unit, the design temperature limits can be exceeded and a catastrophic failure of the unit can result. Applicants have discovered a method of overcoming this obstacle without sacrificing the quality of the oxidized effluent obtained by supercritical wet oxidation.
It is an objective of the invention to control the temperature and the pressure of a wet oxidation system when treating high strength wastewaters. It is a further objective to treat a high strength wastewater without diluting the waste, dilution requiring a larger capacity treatment system. It is also an objective of the invention to provide a treated effluent which can meet stringent discharge requirements.